Setting Sail for the Stars

Setting Sail for the Stars

Cracking the whip and unfurling gray sails are among
new techniques under discussion at the 1999 Advanced Propulsion
Research Workshop

April 8, 1999:
A fixture in any movie involving a trek across the desert is
a store with a sign warning, "Last chance for gas, next
200 miles." Right now, the sign at the edge of the solar
system reads, "Last chance for gas, next 4.3 light years."
If you're using an interstellar sail, you can ignore the sign
and coast to the next solar system. Since the mid-1980s, scientists
have been considering sailing to the stars with a "breeze"
produce by lasers or microwave transmitters.

Right: A "conventional" solar sail, fully deployed
and cruising into interstellar space. Innovative ideas for "gray"
and electromagnetic sails may leave this concept in the interstellar
dust. (NASA)

"A propellant-free system is very attractive because
the main problem with interstellar travel is the weight of the
propellant," said Geoffrey Landis of the Ohio Aerospace Institute
at NASA's Glenn Research Center. He spoke Wednesday morning to
the 10th annual Advanced Propulsion Research Workshop held by
NASA, Marshall, the Jet Propulsion Laboratory, and the American
Institute of Aeronautics and Astronautics being held Tuesday-Thursday
at the University of Alabama in Huntsville.

The original notion of space sails is to unfold
a large aluminum coated Mylar blanket, face it to the sun, and
let sunlight and the solar wind push the craft deeper into space.
But that takes a long time to get anywhere. In 1984, Dr. Robert
Forward, , vice president of Tethers Unlimited in Seattle and
an advocate of sails and tethers for space propulsion, proposed
giving Mother Nature a hand by using high-power laser or microwave
transmitters that would beam for a few days or weeks to speed
the probe on its way.

Gray sails could provide a better ride

Forward's Starwisp concept would have used a mesh of superconducting
aluminum wires to receive its "push" from microwave
photons, and then reflect to produce an equal magnitude thrust.
This would propel the craft from Earth orbit past Neptune, at
1/20th the speed of light, in just a week. Since then, Forward
and others have been rethinking the concept.

"My major message is, that's wrong, don't use it"
said Forward as he pointed at the equation he used in his initial
studies. Since 1984, he has determined that the sail material
would absorb a significant amount of the energy, weakening the
structure and possibly letting it collapse.

Forward now proposes putting that absorption to work in a
"gray sail" made of carbon. The sail would absorb the
light, getting a push from it, and reradiate it as infrared energy.
With the sail oriented properly to the source, this would generate
a significant amount of thrust in the desired direction.

A mission to interstellar space could be accomplished with
a combination sail. An aluminum coating - just 70 atoms thick
- would serve as a traditional reflective solar sail to boost
the spacecraft out of Earth orbit, then cancel its solar orbital
velocity so it plunges on a near-miss trajectory towards the
sun.

Right:
The sunshade for the Next Generation Space Telescope is not as
large as a sail for space propulsion, but will provide valuable
technical lessons on how to build one. (NASA)

As it passes just 3 solar diameters from the sun's visible
surface, the aluminum would evaporate, exposing the carbon structure
underneath. The carbon would absorb sunlight and heat to 2,000
K (almost 3,600 deg. F). Radiating infrared light would accelerate
the craft at 14 times Earth's gravity (the Space Shuttle reaches
a maximum of 3 G during launch).

"The trajectory is nearly a straight line" away
from the sun, Forward said. He is proposing a laboratory demonstration
using a 1 kilowatt microwave beam to levitate a 2.5 cm (1 in.)
square, 02.5 micron-thick carbon film in a vacuum chamber.

Wild & crazy - yet disciplined

Whether you view today as the
"good old days" or "the dark ages," space
transportation has to become cheaper, faster, and more frequent
to really open the "highway to space."

Speaking to the keynote banquet
for the 10th annual Advanced Propulsion Research Workshop, Art
Stephenson the director of NASA's Marshall Space Flight Center
outlined some of the goals for improvements in space transportation.

He said that one goal is to increase
the safety of space travel to be comparable to flights on commercial
airliners within 40 years. Other goals include increasing a vehicle's
life span to 10,000 missions, and reducing the turnaround between
missions to a few hours with a crew of two persons.

"What we're talking about
here is a revolution in space transportation," he said,
with respect to both Earth-to-orbit and orbit-to-deep space propulsion.

Referring to a 1940s-style graphic
shown at the start of his talk, Stephenson said that "We're
in the good old days, looking to the future." He later cautioned
that "We're really in the dark ages" with regard to
what's been done so far to reduce spacecraft mass and improve
transportation.

Some of the advances will come
from a planned series of technology demonstration flights, such
as the X-34 small launcher demonstration, the X-37 hypersonic
flight demonstrator, and the rocket-based combined cycle engine
in 2005. The latter effort supports work on a launcher that would
incorporate a magnetic-levitated sled to accelerate the vehicle
(below), then engines that would work as a rocket, then a ramjet,
then a rocket again, all to reduce weight and flight costs.

The path just starts there. Showing
an artist's concept of the microwave-powered Lightcraft that
NASA/Marshall is partially funding in advanced studies, Stephenson
said, "This is way out thinking. But it's the kind of thinking
we should be doing to get an elevator to low Earth orbit."

The "way out thinking"
is getting favorable reviews. Stephenson said that NASA Administrator
Dan Goldin "complimented Marshall for working on wild and
crazy propulsion concepts. But, we must combine wild and crazy
ideas with disciplined engineering."

Other Propulsion
Stories this week

Apr 6: Ion Propulsion -- 50 Years in the Making
-
The concept of ion propulsion,
currently being demonstrated on the Deep Space 1 mission, goes
back to the very beginning of NASA and beyond. April
6:
Far
Out Space Propulsion Conference Blasts Off - Atoms locked in snow, a teaspoon from the heart
of the sun, and the stuff that drives a starship will be on the
agenda of an advanced space propulsion conference that opens
today in Huntsville.April 7: Darwinian
Design - Survival of
the Fittest SpacecraftApril 7: Coach-class
tickets for space? - Scientists
discuss new ideas for high-performance, low-cost space transportationApril 8: Setting
Sail for the Stars - Cracking
the whip and unfurling gray sails are among new techniques under
discussion at the 1999 Advanced Propulsion Research WorkshopApril 12: Reaching
for the stars - Scientists
examine using antimatter and fusion to propel future spacecraft.April 16:
Riding
the Highways of Light - Science
mimics science fiction as a Rensselaer Professor builds and tests
a working model flying disc. The disc, or "Lightcraft,"
is an early prototype for Earth-friendly spacecraft of the future.Â

Â

A new use for radio

Landis also finds carbon sails attractive in a reworked approach
to Forward's Starwisp concept. Landis proposes using millimeter-wave
radio to push a carbon sail. Millimeter-wave transmitters are
more efficient than lasers, so less power would be needed to
run the system.

A lens to focus the millimeter waves (using techniques similar
to those that steer phased-array radar beams) would only have
to be 185 km wide, as compared to a 50,000 km fresnel lens that
would be required for a system.

The sail itself would be made of carbon fibers, or possibly
with variants of the high-temperature superconductors that have
been in development since the early 1990s. The transmitter technology
already is becoming available through megawatt-power, 1,110 gigahertz
(0.78 mm wavelength) gyrotrons developed for fusion power experiments.

Landis suggested a laboratory demonstration using a 2 cm (4/5th
inch) diameter cone. Shaping it so it would stay on the beam
"is a tricky design problem, but it's a design problem with
a solution," he said.

A precursor space mission, carrying a 1 kg (2.2 lb) payload
on a 10x10-meter sail would take 20 hours to accelerate. In three
weeks, it would pass the orbit of Pluto and continue outward
to the Oort cloud of comets surrounding the solar system. Reaching
a star would take 400 years, so it's only good as a demonstration.

Even closer at hand is a concept to sail without a deploying
a sail, but throwing a switch and generating one around the spacecraft.
In an approach called Mini-Magnetospheric Plasma Propulsion -
or M2P2 - a probe would imitate nature to get the solar wind
to push it into deep-space.

"The enabling technology is pretty much available today,"
said Dr. R.M. Winglee of the University of Washington Winglee
works in the geophysics program which studies the magnetosphere,
the region of space around the Earth where the solar wind is
deflected by the Earth's magnetic field.

Sailing in a bubble

"What we're proposing to do is create a magnetic bubble
to deflect the solar wind," Winglee explained.

Magnetic sails were proposed by Robert Zubrin, inventor of
the Mars Direct concept. Such sails are limited, so Wingless
suggests injecting plasma (ionized gas) that would drag the magnetic
field lines out and generate a bubble 30 to 60 km (18-36 mi)
in diameter.

The magnetic field
of 0.1 Tesla could be generated by a conventional solenoid, and
the helicon plasma source "is amazingly simple." With
a bottle of just 3 kg (1.5 lb) of helium as the plasma fuel,
the magnetic bubble could be operated for three months. The size
of the bubble would expand and contract with variations in the
solar wind, so the force on the 100 kg spacecraft would stay
constant at 1 Newton (about a quarter pound). The 3 kilowatts
of power to run the magnet and plasma generator would be powered
by solar cells.

Left: An artist's concept shows how Earth's magnetic
field deflects the solar wind and forms the immense magnetosphere
around the planet. Scientists may imitate nature and generate
a mini-magnetosphere around a space probe and let the solar wind
accelerate it into deep space. The solar wind exerts no appreciable
push on the Earth because of the Earth's great mass. (NASA)

Winglee calculates the specific impulse (a measure of efficiency),
would be tens of thousands of seconds. That's 10 to 20 times
better than the Space Shuttle Main Engine.

"We can go faster and lighter than anyone else,"
Winglee said.

How fast?

If launched in 2003, M2P2 would go past the heliopause, where
the solar wind runs into the interstellar wind, by 2013. That's
a distance of more than 150 times the distance from the sun to
the Earth. Voyager 1, launched in 1977, will get there in 2019.

Winglee said that adding dust particles to the magnetic bubble
would enhance the thrust, and accelerate the M2P2 even faster
for a mission to another star.

After giving his briefing, Winglee received a glowing recommendation
from sail advocate Forward: "I just love the audacity of
that concept."

Crack the whip to Mars and back

Forward also is closely involved in developing a precise interplanetary
game of "crack the whip" that could send payloads to
the Moon or Mars.

"Our goal is to develop a public transit system in space,"
said Robert Hoyt, president of Tethers Unlimited.

Hoyt and Forward believe that an interlocking, well-timed
series of rotating tethers could carry payloads from low Earth
orbit to the surface of the Moon with almost no rocket power
involved.

People used to the
smoke and fire of rockets may try to decode tether as an acronym
for an exotic rocket. It's not. A tether is a flexible line or
rope connecting two objects. Scientists have known for some years
that if two bodies were rotating at opposite ends of a tether,
they will snap into different orbits if the tether is broken.
That happened with the unfortunate loss of the Tethered Satellite
System when it was flown on the Space Shuttle in 1996. If the
bodies are moving fast enough, one could be sent on its way to
the planets.

Right: At artist's concept traces
the trajectory for a payload dispatched from the HEFT tether
orbiting Earth to the Lunavator Facility that will place it on
the Moon. (Tethers Unlimited)

Under a contract to the NASA Institute for Advanced Concepts,
Tethers Unlimited is defining a Cislunar Tether Transport System.
The first step is appropriately named HEFT - High-strength Electrodynamic
Force Tether, 90 km long in orbit around the Earth. At the other
end of the line is the Lunavator Facility, a 200 km tether -
plus counterbalance and central mass - in orbit around the Moon.

At the start, HEFT has a 1,000 kg payload at one end of the
tether. It can start rotating by momentum exchange with payloads
coming back from the Moon, reeling the tether in, or by using
the tether itself as part of an electric motor (explained in
a few paragraphs).

When the payload is swung out from Earth, HEFT releases and
the payload sails on to the moon. A little bit of rocket power
is used on the way since tidal forces and other effects will
usually require midcourse corrections.

The payload arrives at the Moon, just in time to meet the
Lunavator Facility as its tip is swinging outward. At this point,
the Lunavator orbits well clear of the Moon. To deliver the payload,
the central mass winches its way to the end with the counterbalance.
Now the center of mass is very close to one end of the tether.
The other end, with the payload, swings down to the surface and
deposits the package at zero velocity.

That may sound impossible, but think of the edge of an automobile
tire. It meets the road at zero speed (but is traveling at twice
your car's speed when it rotates to the top). With the package
delivered, the Lunavator Facility redistributes its masses in
preparation for the next arrival. Or, it can pick up a package,
at zero speed, and sling it back to Earth.

Early work on the Cislunar Tether Transport System led Forward
to extend the idea to Mars.

"When Rob Hoyt first started his calculations, he was
throwing the payloads too hard," Forward said. "He
had to slow them down or otherwise they would escape from the
combined Earth-Moon gravity field. After doing some calculations,
I found that ordinary Spectra [the tough, light-weight fishing-line
material used in the tethers] could throw payloads to Mars."

So he started designing for the
next phase, the Mars-Earth Rapid Interplanetary Transport Tether
(MERRITT) system. Forward has made another advance in that the
outbound payload need not reach orbit. It can make an atmospheric
ascent to 150 km (90 mi) where the EarthWhip picks it up and
throws it outward to Mars. Like the Lunavator, the EarthWhip
touches the payload at nearly zero relative speed, the center
of mass shifts to balance the arrangement, and the tether releases
the payload at the right instant to send it to Mars.

Arrival at Mars is the reverse, with the MarsWhip stage dropping
the payload into the Martian atmosphere to glide or parachute
to its destination.

"It will get you in," Forward said, "You don't
need a deorbit propellant." The Martian atmosphere rules
out tethers going directly to the surface, at least for the foreseeable
future.

The trip to Mars could be made in 116 to 162 days, depending
on the speed of the whip tip. With aerobraking to slow the craft
on arrival at Mars and just before contacting the MarsWhip, the
craft can make the trip in as little as 94 days by increasing
the speed of the EarthWhip.

"We have a new idea," Forward said. "It looks
pretty solid, and it looks pretty promising."

A step down before stepping up

The first step on
this trip will be taken in August 2000 when Marshall Space Flight
Center flies the Propulsive Small Expendable Deployer System
(ProSEDS), a 15 km tether that will act like a small electric
motor to lower the orbit of an expended rocket stage faster than
natural atmospheric decay.

"We believe that an electrodynamic tether has a lot of
applications," said Les Johnson, the principal investigator
at NASA/Marshall.

ProSEDS' tether will expose the last 5 km of wire to make
an electrical connection to the plasma (electrified gas) surrounding
the Earth. As the rocket stage (the second stage of a Delta II
that will launch an Air Force satellite) orbits the Earth, the
wire cuts through the Earth's magnetic field. With electronics
on the stage completing the circuit, the tether thus generates
an electrical current at the expense of its speed, thus lowering
its altitude.

"After ProSEDS, there may be a commercialization of this
concept," Johnson said, "with operators putting these
onboard spacecraft to deorbit rocket stages without using fuel."

NASA is also studying an Electrodynamic Tether Upper Stage
that - by proper control of the electrical current - could boost
satellites to higher orbits, then return itself to a lower orbit
to deliver more satellites. A highly profitable application could
be on the International Space Station where a low-cost electrodynamic
tether could save about $1 billion a year in the cost of supplying
reboost propellants.

Spacecraft
may fly on "empty"
(Jan. 22, 1999) Using a propulsive tether concept, spacecraft
may be able to brake or boost their orbits without using onboard
fuel. A NASA/Marshall project, named "ProSEDS," is
slated to demonstrate braking, by accelerating an expended rocket
toward re-entry.